Second generation mobile communication refers to digital transmission and reception of voice (audio), and examples of the communication includes Code Division Multiple Access (CDMA), Global System for Mobile communications (GSM) and the like. General Packet Radio Service (GPRS) evolved from the GSM has been proposed. The GPRS is a technique for provisioning packet switched data services based upon the GSM system.
Third generation mobile communication refers to enabling transmission and reception of images (video) and data as well as voice (audio). Third Generation Partnership Project (3GPP) has developed International Mobile Telecommunication system (IMT-2000).
Various attempts have been made in the second and third generation mobile communications in order to more efficiently use an uplink from a terminal to a base station and a downlink from the base station to the terminal.
As one of the attempts, it has been proposed that audio data (i.e., data based upon circuit switching) generated by two terminals in the uplink and downlink are multiplexed into one radio resource for transmission and reception. For example, for Time Division Multiple Access (TDMA), audio data generated by two terminals are multiplexed into one time slot (Absolute Radio Frequency Channel Number (ARFCN) as a physical sub-channel and TDMA frame) so as to be transmitted and received.
As such, the technique for enabling transmission and reception by multiplexing audio data generated by two terminals into one time slot is referred to as Voice Services over Adaptive Multi-user Channels on One Slot (VAMOS). This technique also enables transmissions and receptions of video data, or general data generated by two terminals into one time slot.
Also, users who share the same physical resource may be called sub-channel users.
The VAMOS is implemented such that user's audio data is sent via a specific channel, for example, a traffic channel (TCH). Here, the specific channel may also be referred to as a multi-user channel. Examples of the TCH used for the VAMOS may include TCH/FS, TCH/HS, TCH/EFS, TCH/AFS, TCH/AHS, TCH/WFS and corresponding related control channels (e.g., FACCH and SACCH). Each of the TCH channels and each of the related control channels are combined in one pair using AQPSK (Adaptive Quadrature Phase Shift Keying) in downlink and GMSK in uplink, thus being mapped into one radio resource.
Hereinafter, description will be given of the related art with reference to FIGS. 1 to 3.
FIG. 1 is a block diagram illustrating modulation of sub-channels in the related art VAMOS.
Referring to FIG. 1, a sub-channel ai for first user's data, a sub-channel bi for second user's data and a control signal for power adjustment are mapped into quaternary symbols, at a transmitting end, i.e., a base station, and then undergo a pulse shaping after a symbol rotation.
That is, the data of the two sub-channels ai and bi are coded to be mapped to the AQPSK symbols.
FIG. 2 is a flowchart illustrating a method for pairing two terminals in the related art VAMOS, and FIG. 3 is an exemplary view of an application of the method shown in FIG. 2.
As shown in FIGS. 2 and 3, when transmission and reception of audio data are performed between several terminals and a base station (or a network entity), the base station (or a network entity) selects two appropriate terminals (S11).
The base station (or a network entity) then adjusts power of the selected two terminals (S12), thereby preparing a pairing operation. That is, the base station (or a network entity) adjusts power of the two terminals such that link qualities of the selected terminals can meet a target Signal to Noise Ratio (SNR).
If the link qualities of the selected terminals meet the target SNR, the base station (or a network entity) sends a pairing command to the two terminals (S13). Accordingly, as shown in FIG. 3(c), one of the terminals performs an intra-cell handover (or an inter-cell handover) so as to use a time slot which is being used by another terminal.
As mentioned above, for pairing two terminals, power of the two terminals should reach an appropriate level so as to satisfy a target SNR. However, there is a difficulty in maintaining Frame Error Rate (RER) and Co-channel interference between the two terminals at a satiable level while maintaining the powers of the two terminals at appropriate levels. Furthermore, since there is no way to recognize an accurate level of power suitable for satisfying both FER and the co-channel interference, this situation may have a very high possibility of call-drop occurring between the paired two terminals.
As such, there is such high possibility of the call-drop occurrence of the paired two terminals, especially, the call-drop problem is made worse in a fading environment. So, to prevent the call-drop, a margin of an appropriate power level should be set. However, another problem is raised because it cannot be known how high margin of the power level is to be set.
Also, in order to ensure link qualities after pairing, it should be determined how high level of initial power imbalance of the two terminals is to be set.
Meanwhile, to solve those problems, several approaches have been introduced, such as pairing terminals with high SNRs, increasing power prior to pairing, and the like. However, the link qualities after pairing cannot still be ensured, there still remains the call-drop occurrence in the fading environment.
In order to overcome the problem, the method shown in FIG. 4 has been proposed.
FIG. 4 is a flowchart illustrating another method for pairing two terminals in the related art VAMOS.
As shown in FIG. 4, when transmission and reception of audio data are performed between several terminals and a base station (or a network entity), the base station (or a network entity) selects two appropriate terminals (S21).
The base station adjusts powers of the selected two terminals (S22), thereby preparing a pairing operation. That is, the base station adjusts power of the two terminals such that link qualities of the selected terminals can meet a target SNR.
If the link qualities of the selected terminals meet the target SNR, the base station (or a network entity) sends a pairing command to the two terminals (S23). Accordingly, pairing is performed between the two terminals such that one of the two terminals can use a time slot which is being used by another terminal.
Here, the one terminal transmits and receives audio data by using both the paired time slot and a previously used time slot (S24). For example, if the first terminal having used the second time slot is paired with the second terminal to use the fourth time slot which is being used by the second terminal, the first terminal transmits and receives audio data by use of both the second and fourth time slots.
Afterwards, it is determined whether or not the link quality of the paired time slot is satiable (S25), and if not, the pairing is released (S26). However, if the link quality of the paired time slot is satisfactory, the resource of the previous time slot is released (S27), and audio data transmission and reception are continued by use of the paired time slot.
As described above, the another method according to the related art shown in FIG. 4 has been proposed to prevent the occurrence of call drop in the fading environment, but still has several problems as follows.
First, in order to maintain the previous time slot after the pairing, the base station (or a network entity) should be able to allocate multiple slots to one terminal. Also, in order to allocate multiple slots to one terminal, a control signal should be sent and received, resulting in disabling an efficient use of network resources.
In addition, the terminal has to use both the paired time slot and the previous time slot at the same time, which is difficult to be technically implemented. That is, the terminal within one frame should use several time slots, however, it cannot be easily implemented. In particular, if frequencies of the time slots are different, the terminal should perform frequency hopping for using several time slots, however, it is also very difficult to be implemented. Furthermore, if an interval between time slots is narrow, the terminal may not take sufficient time for the frequency hopping.